Octane Springs is a large oil field in mid-eastern Bone County, San Andreas. Its pumps and derricks, which can be easily spotted from the nearby tracks of the Brown Streak Railroad, service Green Palms, the refinery located immediately to the west. It has exactly 13 pumps. The Octane Springs area used to be completely grassy; it can be noticed around the pumps. To the northwest of Octane Springs is Area 69, while to the south is Hunter Quarry. Meanwhile, east of the oil field is Las Venturas, specifically the Blackfield district. Many of the moving components on the oil rigs are not solid and the player will pass right through them.
This is most likely the source of the fuel sold by Tosser, Gasso, Going and Xoomer, which is then shipped to the nearby Green Palms refinery via the pipelines that can be seen in-game.
Grand Theft Auto V
Similar looking oil fields can be seen in the GTA V Trailer at 1:03. However, the Los Santos Skyline can be seen fairly close in the background, so it's unlikely that they will be the same fields. Its pumps read brute.